This invention relates to solvent resistant polyimidesiloxanes. In particular, it relates to polyimidesiloxanes prepared from a mixture of non-siloxane diamines, which impart solvent resistance without a loss of other desirable properties.
Polymers used in coating wire and cable are required to have a low dielectric constant, high elongation, and resistance to attack by chemicals and solvents. Polymers used in making three dimensional circuit boards (known as 3-D molded wire boards) are required to have similar properties and, in addition, need to be solderable. A polymer is solderable if it will not turn soft and/or decompose at the soldering temperature, which is about 260.degree. C. Polymers used in these applications have included polyvinyl chloride, polyfluorovinylidene, polyethersulfones, polysulfones, and polyetherketones. However, the use of halogenated polymers is being restricted because they emit toxic gases when burned. While the sulfone and ketone polymers have excellent properties, their dielectric constants are in the range of about 3.4 to about 3.6, and, for many applications, a coating having a lower dielectric constant would be desirable.
Because polyimidesiloxanes have many of these desirable properties, including a lower dielectric constant and high temperature stability, they are likely candidates for use in coating wire, cable, and circuit boards. However, it is has been found that conventional polyimidesiloxanes exhibit poor solvent resistance. When conventional polyimidesiloxane coatings are cleaned with solvents such as methylene chloride, trichloroethane, or methyl ethyl ketone, which are used in the wire and cable and circuit board industries, the polyimidesiloxanes will typically lose weight as they dissolve in the solvent, or will gain weight as they absorb the solvent. This lack of solvent stability results in the weakening of the coating and its subsequent cracking and failure. Efforts to make polyimidesiloxanes less subject to solvent attack have, until now, resulted in a deterioration of their other desirable and necessary properties, such as elongation.